Officials are probing how a 51-year-old highway bridge came to collapse in the Italian port city of Genoa yesterday, killing at least 26 people and injuring 16 others as it sent dozens of vehicles tumbling into a heap of concrete and twisted steel.

Aussies tell of Berlin market crash horror

Australians in Berlin have expressed shock and grief at the deaths of 12 people killed when a truck ploughed into a busy Christmas market in the German capital.

Many of the Aussies had either been to the market outside the historic Kaiser-Wilhelm Memorial Church in the city's central shopping district or were staying at nearby hotels when the crash happened on Monday night, local time.

Trisha O'Neill was across from the market drinking gluhwein with friends when suddenly there were "blood and bodies everywhere".

"I just saw this huge black truck speeding through the markets crushing so many people and then all the lights went out and everything was destroyed," she told the ABC.

"I could hear screaming and then we all froze."

Ms O'Neill, who moved to Germany in September, burst into tears when she saw how many people, including children, were laying on the ground.

"I never thought I would see something like this in my life," she said.

German police are unsure whether terrorists were involved in the crash or if it was a tragic accident.

However they have arrested one man, believed to have been the driver, while a Polish man, a passenger, died at the scene.

Forty eight people were injured in the crash and continue to be treated at hospitals across the city.

Police armed with machine guns patrolled streets surrounding the church, which is one of Berlin's most famous landmarks and known as an anti-war memorial to peace and reconciliation.

Meanwhile, Australian government officials are trying to establish whether any Australians were injured or killed.

Aussies in Berlin took to social media to reassure family and friends back home that they were safe.

Brisbane couple Bo Nicholson and Emma Andrews had been at the market just an hour before the deadly crash.

"Very sad for those that have died, but don't worry about us. Just deeply saddened by the loss for the families at this time of year - this will scar this city," they wrote on Facebook.

Melbourne couple Max Nolan and Laura Nicol were left shaken when they heard about the crash at their Berlin hotel.

"It's such a beautiful city and the Christmas markets bring so much positivity to the whole city that it is shocking some members of the human race could be capable of committing such an unthinkable act," they wrote on Facebook.

The Department of Foreign Affairs is warning Australian travellers to avoid the area and remain vigilant in public places.

The overall level of advice for Germany remains unchanged, with travellers advised to exercise normal safety precautions and be aware of the increased threat of terrorism.

Prime Minister Malcolm Turnbull said while Australia would do all it could to reduce the chance of terrorist attack here, he conceded "we cannot mitigate all risk entirely".

"Everyone is well prepared to accept to anticipate attacks of this kind," he said.

Anyone concerned about the welfare of Australians in Germany can call DFAT on 1300 555 135 or the embassy in Berlin on 03 0880 0880.